I recently purchased two diamond sharpening stones from amazon. One of them was a medium grit (600) and the other was a fine grit (1200). They arrived in the mail about a week ago. When I unboxed them I noticed that neither one was labeled for its specific grit, and to the touch, they both feel exactly the same.... How do I figure out which is which? I haven't used them yet... Would I be better suited just giving them both try and see which one puts a finer polish on my chisels? I guess that should tell me, but any suggestions would be greatly appreciated. By the way, they are not the DMX diamond sharpening stones, they were the "Ultra Sharp II Diamond Stone Kit - Medium and Extra Fine". In fact, I am actually looking at the product on Amazon right now, and the one's in their picture clearly are easy to tell apart just by looking at them.... Do you think they might have sent me two stones with the same grit by mistake?

The photo's on Amazon vary from listing to listing. On some sets, the course and medium look similar and the extra fine is very smooth. On others, the medium and extra fine look the same with only the course appearing rough. I think it's likely the photos are not necessarily of the exact stones for whatever reason. I would return them since they are not labelled which is awfully odd considering these are not inexpensive items and easily mixed up.
– OttoNov 22 '17 at 16:10

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Yes you hit the nail on the head, rub some steel on them and compare the scratches. There is unfortunately the chance that you did get two of the same grit, but if they are the grits they're sold as there should be a clear difference in the results on steel even if the plate surfaces themselves look very much alike.
– GraphusNov 22 '17 at 17:49

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FWIW I don't think 600 is a good medium grit, that's the 'fine' in some basic kits! I think 320 is about right for a medium, and much lower than 240 for a coarser one. Something closer to 150 is far preferable for the lowest grit in a three-plate progression, so if you're thinking of getting a coarse I'd go for 100, 120 or 150 and nothing finer.
– GraphusNov 22 '17 at 17:50

I don't know about this brand but my DMT plates came with a notice saying they will seem rough at first.
– StevenNov 22 '17 at 22:55

@Steven Yes all diamond plates have a break-in period where larger clumps and/or atypically high particles need to broken down or broken off. Manufacturers mentioning this is a good thing since it's obviously something every potential user needs to know, but they tend to fall down in given an unrealistic timeframe or usage period for this. One says it takes only "a few uses" but meanwhile, in the real world, this is found to be ridiculously understating it LOL
– GraphusNov 23 '17 at 19:42

2 Answers
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If I were in your situation, trying to figure out the grit of some unmarked diamond stones, I would get the iron out of my plane, put it in the angle guide, and mark the edge of the blade with a sharpie.

Then I'd see how many strokes it took to remove the sharpie, and I'd check frequently to see if I could discern any linear marks from the abrasive. On coarser grits you will see some linear marks but at high polish grits you'll just get a mirror surface.

Compare the results with some different grits of sandpaper and you should be able to get a reasonable idea of how the stones will function, if not precise grit counts.

Some diamond plates use color to identify. Lacking that use a Jewller's loupe (magnifying eye piece). I must say years ago I moved to using Japanese water stones. They are made by King as well as others. NOTHING else works better. They cut fast in coarser grades ( 700 - 1200) and polish to a mirror in fine grades ( 5000 - 10000 ) even with years of experience I found they are better than Diamond. One reason is they slowly wear away into a thick slurry which not only evens and lubricates the cutting, but constantly exposes new sharp grit.